54 research outputs found
Multiband Transport in Bilayer Graphene at High Carrier Densities
We report a multiband transport study of bilayer graphene at high carrier
densities. Employing a poly(ethylene)oxide-CsClO solid polymer electrolyte
gate we demonstrate the filling of the high energy subbands in bilayer graphene
samples at carrier densities cm. We observe a
sudden increase of resistance and the onset of a second family of Shubnikov de
Haas (SdH) oscillations as these high energy subbands are populated. From
simultaneous Hall and magnetoresistance measurements together with SdH
oscillations in the multiband conduction regime, we deduce the carrier
densities and mobilities for the higher energy bands separately and find the
mobilities to be at least a factor of two higher than those in the low energy
bands
High-Responsivity Graphene-Boron Nitride Photodetector and Autocorrelator in a Silicon Photonic Integrated Circuit
Graphene and other two-dimensional (2D) materials have emerged as promising
materials for broadband and ultrafast photodetection and optical modulation.
These optoelectronic capabilities can augment complementary
metal-oxide-semiconductor (CMOS) devices for high-speed and low-power optical
interconnects. Here, we demonstrate an on-chip ultrafast photodetector based on
a two-dimensional heterostructure consisting of high-quality graphene
encapsulated in hexagonal boron nitride. Coupled to the optical mode of a
silicon waveguide, this 2D heterostructure-based photodetector exhibits a
maximum responsivity of 0.36 A/W and high-speed operation with a 3 dB cut-off
at 42 GHz. From photocurrent measurements as a function of the top-gate and
source-drain voltages, we conclude that the photoresponse is consistent with
hot electron mediated effects. At moderate peak powers above 50 mW, we observe
a saturating photocurrent consistent with the mechanisms of electron-phonon
supercollision cooling. This nonlinear photoresponse enables optical on-chip
autocorrelation measurements with picosecond-scale timing resolution and
exceptionally low peak powers
Li Intercalation into Graphite: Direct Optical Imaging and Cahn–Hilliard Reaction Dynamics
Lithium intercalation into graphite is a critical process in energy storage technology. Studies of Li intercalation kinetics have proved challenging due to structural and phase complexity, and sample heterogeneity. Here we report direct time- and space-resolved, all-optical measurement of Li intercalation. We use a single crystal graphite electrode with lithographically defined disc geometry. All-optical, Raman and reflectance measurements distinguish the intrinsic intercalation process from side reactions, and provide new insight into the microscopic intercalation process. The recently proposed Cahn–Hilliard reaction (CHR) theory quantitatively captures the observed phase front spatial patterns and dynamics, using a two-layer free-energy model with novel, generalized Butler–Volmer kinetics. This approach unites Cahn–Hilliard and electrochemical kinetics, using a thermodynamically consistent description of the Li injection reaction at the crystal edge that involves a cooperative opening of graphene planes. The excellent agreement between experiment and theory presented here, with single-crystal resolution, provides strong support for the CHR theory of solid-state reactions.United States. Dept. of Energy. Office of Basic Energy Sciences (DE-SC0001085
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